Certain polymer compositions have long been recognized by those skilled in the art as additives useful in reducing fluid loss. These compositions are commonly referred to as "fluid loss additives".
Fluid loss additives are typically used in cementing oil wells. For oilfield cementing operations to be successful additives which reduce fluid loss are required to be added to the cement. Such additives will be used in well operations where the bottom hole circulating temperature (BHCT) may range from 80.degree.-400.degree. F., substantial salt concentrations may be present, and cement slurry retardation and viscosity are critical aspects as the same affect pumpability and compressive strength.
Some examples of fluid loss additives are set forth in the following U.S. Pat. Nos.: 4,678,591 (Giddings et al.), issued Jul. 7, 1987, 4,515,635 (Rao et al.), issued May 7, 1985, and 4,676,317, (Fry et al.), issued Jun. 30, 1987.
The Rao patent discloses typical polymers used as fluid loss additives, e.g., copolymers of N,N-dimethylacrylamide and 2-acrylamido-2-methylpropanesulfonic acid having molar ratios of between 1:4 and 4:1, respectively, and an average molecular weight of between about 75,000 and about 300,000. Other fluid loss additives disclosed in the Rao patent are: a copolymer of a sulfonic acid modified acrylamide and a polyvinyl crosslinking agent, and a hydrolyzed copolymer of acrylamide and 2-acrylamido-2-methylpropanesulfonic acid (AMPS).
Giddings et al. disclose a terpolymer composition for aqueous drilling fluids comprising sodium AMPS (72-3.8%), N,N-dimethylacrylamide (13.5-0.7%), and acrylonitrile (9.5-0.5%). This terpolymer composition also contains lignin, modified lignin, brown coal, or modified brown coal in an amount ranging from between 5-95% with the lignin, modified lignin, brown coal, or modified brown coal having been present during the polymerization of the water-soluble polymer. It has a molecular weight in the range of 10,000-500,000. The lignin, modified lignin, brown coal, or modified brown coal is from the group consisting of lignites, sulfonated lignites, lignins, leonardites, lignosulfonates, alkali metal humic acid salts, humic acids, and sulfonated humic acids.
Another fluid loss additive disclosed in Giddings et al. is RESINEX, a sulfonated lignite complexed with sulfonated phenolic resin.
The Fry patent discloses a graft polymer fluid loss additive comprising a backbone of at least one member selected from the group consisting of lignin, lignite, and their salts, and a grafted pendant group of at least one member selected from the group consisting of AMPS, acrylonitrile, N.N-dimethylacrylamide, acrylic acid, N,N-dialkylaminoethyl methacrylate, wherein the alkyl radical comprises at least one member selected from the group consisting of methyl, ethyl and propyl radicals.
The petroleum industry prefers a fluid loss additive that has as little effect on compressive strength, set time, viscosity, and thickening time as possible; a fluid loss additive that is salt tolerant (i.e., does not exhibit substantial loss of effectiveness in the presence of salt); and a fluid loss additive that is chemically stable during cementing operations. Furthermore, fluid loss additives should be compatible with as many other additives and environmental conditions as possible, should be soluble in cement slurries at normal ambient temperatures encountered in oil well cementing operations, and should continue to provide fluid loss characteristics over broad temperature and cement pH ranges.
The present inventors have discovered that fluid loss may be reduced in oilfield cement slurries by using novel vinyl grafted wattle tannin fluid loss additives which encompass much of the aforementioned desired characteristics.
U.S. Pat. No. 4,579,927 (Patel et al.), which issued on Apr. 1, 1986, discloses a water soluble polymer, i.e., a polymer consisting of a minor portion of tannin derived monomers and one or more acrylic monomers, which exhibits thermal stability characteristics when used as an additive in aqueous drilling fluids. These water soluble polymers are purported to control the viscosity, gel strength and fluid loss characteristics of an aqueous drilling fluid when exposed to downhole temperatures in excess of 300.degree. F. The water soluble polymers are typically copolymers of flavanoid tannins and ethylenically unsaturated comonomers. The polymers are formed by free radical initiated polymerization.
Some examples of the water soluble polymer are (1) 20% sulfonated quebracho tannin, 70% AMPS and 10% acrylamide, and (2) 11% sulfonated quebracho tannin, 54% acrylamide, 21% AMPS, and 13% maleic acid. The incorporation of AMPS into the polymer improves the rheology and fluid loss of the drilling fluid. Common sources of flavanoid tannins include extracts of quebracho, wattle, mimosa, mangrove, chestnut, gambier and cutch. In certain applications the flavanoid tannins are sulfonated to enhance solubility characteristics.
All the examples in Patel et al. are directed to sulfonated quebracho tannins. In comparative studies conducted by the present inventors, it was apparent that the novel vinyl grafted wattle tannins of the present invention clearly out performed the quebracho tannins as fluid loss additives in cement slurries. Tannin and acrylamide retard the set time of cement slurries. According to the present invention, the amount of tannin and acrylamide was adjusted to be useful for fluid loss but not overly retard the set time. The data generated by the present inventors demonstrate that the grafted quebracho tannin examples set forth in the Patel et al. patent would not be useful as fluid loss additives for oilfield cementing.
It is noted that the vinyl grafted quebracho tannins of Patel et al. were prepared primarily for use as fluid loss additives in drilling fluids or muds. The application of such fluid loss additives to hydraulic cement, although briefly referred to in Patel et al., was apparently not attempted. Had Patel et al. tried to use the fluid loss additives formed in accordance with the teachings thereof in oilfield cement slurries, it is believed those additives would not have exhibited acceptable fluid loss properties.
The present inventors have discovered through extensive experimentation and comparative analysis that wattle tannin, not quebracho tannin, grafted to selected vinyl monomers performed extremely well as fluid loss additives in cement slurries. Furthermore, when oilfield cement slurries were treated with the vinyl grafted wattle tannins of the present invention they exhibited improved fluid loss control over the corresponding vinyl polymers alone and vinyl monomers grafted with lignite.
The development of a vinyl grafted wattle tannin fluid loss additive that was repeatable and which exhibited satisfactory fluid loss performance characteristics was extremely difficult. Many graft reactions with tannin resins were performed by the inventors. Some resins were made by reacting tannin with formalin under acid and base conditions. Some resins were made with epichlorohydrin and glutaraldehyde under appropriate conditions. Acceptable grafts were only realized when the tannin level was cut back from 20%. The level of initiator required for grafting was proportional to the tannin content. Only when the tannin level was reduced to 14% and below did the grafts become repeatable and the performance improved.
Additional advantages of the present invention shall become apparent as described below.